The scale on a map is 8cm : 10km. If the distance between two cities is 50km, how far apart in cm are the two cities on the map?
Explanation: The scale means that every 8cm on the map corresponds to an actual distance of 10km. An actual distance of 50km is the same as 5 $\cdot$ 10km. The distance between the two cities on the map is 5 $\cdot$ 8cm, or 40cm.